Waste heat recycling system

ABSTRACT

A heat exchanger heats fresh air moving to a product dryer by running the fresh air around hot exhaust tubes which are discharging from the dryer. Detergent sprays may be positioned to direct cleaning fluid into the tubes. The fresh air is introduced into a pre-dryer through pipes having nozzles which are at 30° angles to the product path.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to dryers, especially fabric dryers, and, moreparticularly, to means for reducing the fuel consumption necessary toheat the dryers increasing production of the dryers, and to reducing theemission of pollutants into the air from the exhaust of the dryers.

2. Summary of the Invention

It is an object of this invention to provide an energy saving system fora conventional dryer of fabric or other products.

It is another object of this invention to provide an efficient heatexchanger for heating air introduced into a product dryer.

It is still another object of this invention to combine an air heaterwith an exhaust pollutant remover.

Still another object is to increase production rates of a known dryer bysimultaneously reducing the amount of energy necessary to heat thedryer.

Basically, these objects are obtained by combining on a conventionaldryer a fresh air chamber in which fresh air is forced along acircuitous path. Tubes for removing the hot exhaust from the dryer arepositioned to be exposed to the air in said circuitous path so that theexhaust is cooled simultaneously with the increase in temperature of thefresh air. The most desirable advantages from this system are that asignificant amount of heat is recovered and transferred into the freshair and thus reduces the amount of fuel consumed in bringing the air upto drying temperature. Secondly, this waste heat allows an increase inproduction rates of the dryer due to pre-drying without an increase inenergy consumption. Thirdly, reducing the temperature of the exhaustgases facilitates removal of moisture and polluting particles. Fourthly,the pre-drying at a temperature generally intermediate room temperatureand dryer temperature gives an improved feel and finish to the fabric.

In embodiments in which the exhaust collects in the tubes, the exhausttubes are cleaned by being sprayed with a cleaning solution which helpsto keep the interior of the tubes free from buildup and optimizes heattransfer between the exhaust gases and the fresh air. Also in thepreferred embodiment the heated fresh air is introduced into thepre-dryer through pipes having nozzles which lie at angles ofapproximately 30° to the vertical above and below the path of the movingproduct. In this way the air flows more smoothly along the productproviding better heating and less vibration to the product. Preferably aportion of the heated air can also be diverted as required to supplymake-up air to the main dryer sections also.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWING

FIG. 1 is an isometric of a preferred form of the invention with partsbroken away for clarity.

FIG. 2 is a fragmentary isometric of a portion of the pre-dryer shown inFIG. 1.

FIG. 3 is a horizontal section taken along the line 3--3 of FIG. 1.

FIG. 4 is a fragmentary, vertical section taken along the line 4--4 ofFIG. 3.

FIG. 5 is a fragmentary, vertical section taken along the line 5--5 ofFIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

As best shown in FIG. 1, a dryer 10 includes a pre-dryer section 12 anda main dryer section or sections 14. The pre-dryer and dryer sectionsare fabricated from three-inch thick, insulated panels to receive fabricor other product. Drying temperatures of approximately 300° F. areobtained in the main dryer sections by heating with gas, oil, etc. Inthe apparatus illustrated, a conveyor such as moving rails 16 carries, awet synthetic fabric along a path P with the fabric impaled on uprightpins on the rails in a conventional manner. The path passes into thepre-dryer through a transverse slot 12a in the pre-dryer and thencethrough a second transverse slot 12b between the pre-dryer and the maindryer sections. As is well understood, the fabric departing from theopposite end of the dryer will be dry and free of volatile fluids.

The heat recycling apparatus of this invention includes a chamber 18having a plurality of baffle or divider plates 20a,20b,20c which directfresh air from an inlet 22 along a circuitous path to a fresh air outletduct 24. Positioned vertically within the chamber and exposed to thefresh air are a plurality of modular tube sections 25 each having aplurality of approximately two-inch diameter aluminum tubes 26 eachabout twelve feet in length. The tubes connect with an exhaust outletheader 28 and an exhaust inlet header 30. Positioned within each of theheaders are a plurality of cleaning solution nozzles 32 which directdetergent spray into the tubes for cleaning the tubes and maintainingtheir high heat transferability. It should be understood, however, thatthese sprays are needed only where the exhaust is of a type that tendsto clog the exhaust tubes.

The exhaust inlet header for the hot exhaust tubes connects to a duct 34that communicates with various main dryer sections 14 to remove the hotexhaust at a temperature of about 300° F. from the dryer sections.Preferably the duct 34 is at an angle to the horizontal so thatcondensation will flow along the duct to be discharged through a draintube 40.

Fresh air heated to a temperature of about 185° F. by the exhaust gasesleaves the chamber 18 through the duct 24 and is drawn through thechamber and forced into the pre-dryer 12 by a conventional centrifugalfan 42 driven by a motor 43. The fan directs most of the heated air intopipes 44 which are extended transversely of the product path P and aboveand below the path. The pipes are provided with nozzles or orifices 46which are at an approximately 15° angle with respect to the vertical.The 15° angle directs the air at an angle to the fabric to avoidbouncing the fabric off the pins on the rails 16 and more adequatelywipe the air along the fabric for more even drying action. The pipes arespaced approximately 12 to 15 inches apart and can vary in diameterbased on dryer width and air volumes. Each pipe is fed from the main fan42 and is provided with a damper 47 for individual air volume control.The pre-dryer also has an exhaust fan 48 to remove excess air that hasnot been sucked into the dryer slot 12b. This exhaust fan 48 has a unitcomprising a volume control damper, face damper, and a bypass damper soexhaust air from the pre-dryer can either be thrown out into theatmosphere through a stack 48a or directed towards the ceiling of theplant for winter heat purposes through a vent 48b. This unit can beautomatically controlled by a temperature thermostat sensing outdoortemperature to determine whether to discharge to atmosphere or to insidethe plant. In the 185° F. preheat the fabric does not get heated to apoint to drive off significant pollutant gases.

The fan 42 also communicates with a make-up duct 54 which is providedwith dampers 55 for each of the main dryer sections to provide make-upair to the dryer sections as desired. Only enough make-up air is used inthe main dryer sections to prevent dryer air unbalance conditions whichis accomplished by means of the volume control dampers for each make-upair point in the dryer sections. Dryer air unbalanced conditions occurswhen excess recirculated air is discharged or excess fresh air isintroduced to drastically change the normal operating pressures withinthe main dryer sections.

In operation of a dryer exhausting air at 300° F., the heat exchangerwill reclaim about 45 to 55 percent of the waste heat. That is, when anexhaust of approximately 10,000 CFM at 300° F. air is put into the tubes26, the exhaust contains approximately 1,732,000 BTU/HR. Whencalculating on the basis of SCFM, that is air at 70° F., then 10,000ACFM at 300° F. converted to SCFM equals: ##EQU1## heat available inexhaust stream.

If the outside air temperature is 70° F. the cold air stream calculatedfor 7,000 SCFM returning to the pre-dryer will deliver from 779,400BTU/HR. to 952,600 BTU/HR. Therefor the efficiency will be 45 to 55percent.

This 10,000 CFM at 185° F. will be directed to the supply fan 42 whichwill force some of the heated air to the existing dryer compartments tobe used as make-up air as mentioned above. By this method the dryer willconserve fuel usage due to its make-up air being introduced at 180° F.rather than at 70° F. The difference in heat energy is saved by theability of the dryer heat source to use less BTU to reach operatingconditions.

Since the air in the pre-dryer is about 185° F. it can be seen that thefabric is more gradually brought from room temperature of about 80° F.to dryer temperature of about 300° F. by passing through this region ofan intermediate temperature. Such gradual heating reduces thermal shockon the fabric resulting in a better feel to the dried fabric.

The heat exchanger tubes 22 are all in welded, modular units25a,25b,25c. That is, the tubes consist of several completelyindependent sections sealed to prevent cross leakage. The use of modulartube construction allows one section to be replaced independent of theother. Due to expansion of the sections, a unique system is designed forholding the tubes. Each module is separated at the top side plates 60 bymeans of a saw cut 61. This allows the top header to expand. The tubesare kept in line by means of the divider plate sheets 20b and 20c. Aflexible neoprene strip 62 is then secured to all modules after assemblyinto place in the chamber 18 to allow for expansion and also for a tightseal. The top plenum or header 28 and any connecting duct work is alsoseparated by a saw cut 63 and must have a flexible joint so as not tohave too rigid a construction and be able to allow the unit to expand.The joints must all be seamed by bolting at close spacing and the use ofsilicon high temperature gaskets and caulking. The joints need thisspecial attention so as not to leak contaminated air into the fresh airstream.

If desired, a precipitator 80 or other contaminant removal apparatus canbe attached to the outlet end of the header 28 to completely clean theexhaust.

While the preferred embodiment of the invention has been illustrated anddescribed it should be understood that variations will be apparent toone skilled in the art without departing from the principles expressedherein. Accordingly, the invention is not to be limited to the specificembodiment illustrated.

The embodiments of the invention in which a particular property orprivilege is claimed are defined as follows:
 1. Heat recycling apparatusfor hot air dryers of the type having a pre-dryer section followed by amain dryer section each of which comprises a hollow insulated chamberthrough which product to be dried is carried along a path by a movingconveyor, the improvement comprising:a chamber for receiving fresh airto be introduced into the dryer, said chamber including baffle means fordirecting the fresh air from an air inlet to an air outlet through acircuitous multi-pass route within said chamber, a plurality ofvertically positioned hot exhaust tubes within said chamber and exposedin said circuitous route, said hot exhaust tubes having inlet and outletends, said tube outlet ends being adjacent said fresh air inlet, saidvertically positioned hot exhaust tubes including modular sections oftubes each modular section being mounted independent of the next wherebysections can be replaced as modules, said tubes including headers havingcleaning spray nozzles directed upwardly and downwardly into said tubesfor cleaning the interior of the tubes, means for forcing the fresh airalong said route and into said pre-dryer section for extracting heatfrom the exhaust gases and increasing the temperature of the pre-dryersection so as to preheat the product to reduce time in the main dryersection, means for forcing hot exhaust from said main dryer sectionthrough said tubes for heating the fresh air prior to entering thepre-dryer section.
 2. The apparatus of claim 1, said pre-dryer sectionincluding a set of air pipes lying transversely of the product path,said pipes having outlet nozzles directed toward said product path atabout a 15 degree angle to vertical.
 3. The apparatus of claim 1,including means for directing heated fresh air to the main dryer sectionfor use as make-up air.
 4. The apparatus of claim 1, said exhaust tubesbeing mounted for movement relative to the chamber whereby the tubes canexpand when heated.
 5. The apparatus of claim 1, including means fordirecting heated air from said pre-dryer section selectively toatmosphere or to the area around the pre-dryer.